Indian almond leaf and Oak leaf both release an organic humic acid and tannins and a lot of other chemical compounds that can be beneficial for the Betta....they can help the immune response, ease stress, toughen scales/fins, lowers pH(slight-depends on KH/GH), infusoria development and contain antibacterial/fungal properties.

When used for medication you don't want to steep the leaves in water temp overs 110F...since the high temp can compromise the antibacterial/fungal properties.

The tannins can color/stain the water from a light tea hue to very dark amber